Fatty acid-based materials (fatty materials) such as glyceride oils, wax esters, milk fat, and other fatty acid compounds have a long history of use since many of these materials are naturally derived from plants (e.g., vegetable oils) or animals (e.g., tallow, milk fat, fish oil, etc.).
While these fatty materials often have been directly used in their crude state, for use in modern commercial products, these materials are typically subjected to a refining process. Refining processes may be used to remove various contaminants and impurities which are undesirable for reasons of health, performance, aesthetics, etc.
The fatty material may contain impurities such as color bodies, chlorophyll, phospholipids (phosphatides), trace metals, free fatty acids, gums, soaps, and/or other impurities. This variety of diverse impurities has led to the development of numerous refining processes involving particular combinations of chemical and/or physical treatment steps. A detailed review of refining processes for removal of these impurities may be found in the "Handbook for Soy Oil Processing and Utilization," David R. Erikson et al. (ed.), ASA/AOCS Monograph (1980).
Fatty materials may also comprise sulfur, either in the form of naturally occurring sulfur compounds or in the form of contaminants from various processing or refining steps. For example, certain glyceride oils, most notably canola and rapeseed oils are known to contain small amounts of sulfur in the form of episulfides, isothiocyanates, thiocyanates, oxazolidinethiones, sulfates and sulfur-containing fatty acids. These oil soluble sulfur compounds are the product of enzymatic decomposition of sulfur-containing glucosinolates in the plant seed, which occurs during processing of the seed. Fish oils are also known to comprise naturally occurring sulfur-containing compounds. Substantial proportions of sulfur are removed in degumming and alkali refining operations, but refined canola oils, for example, may still contain up to 9 or more ppm sulfur.
Sulfur compounds present both aesthetic and refining problems, They are implicated in the production of unpleasant odors during heating of the oils or other fatty materials. In addition, these sulfur compounds poison the catalysts used during hydrogenation, resulting in either increased catalyst usage (with a corresponding increase in the disposal burden) or longer hydrogenation times resulting in lower production rates. This is an economically important consideration, since enormous quantities of fatty materials are hydrogenated, a reaction in which some of the double bonds are removed in order to alter the material's viscosity (e.g., converting canola oil into margarine). Sulfur has not been found to be removed during conventional refining or oil treatment methods to sufficient extents to avoid problems in hydrogenation. In fact, total sulfur content may increase during treatment with activated bleaching earth (ABE).
One effect of the poisoning of nickel hydrogenation catalysts by sulfur is a shift in selectivity toward increased isomerization of triglyceride double bonds from the naturally occurring cis olefin isomer to non-naturally occurring trans olefin isomer. This reaction is thought to occur when a triglyceride fatty material adsorbs on the catalyst but is not hydrogenated before desorption. The increased trans isomer content typically raises the melting point but also has been cited as a health issue relative to the more naturally occurring cis isomer.
The presence of cis and trans isomers can be studied by infrared spectroscopy while the level of unsaturation can be quantified by NMR techniques. Thus, the ratio of cis-to-trans can be calculated and compared at a constant level of double bond hydrogenation. Higher cis/trans ratios would be observed when a catalyst was less sulfur poisoned. Thus, one possible result of adsorptive sulfur removal prior to hydrogenation would be less trans isomer and therefore a higher cis/trans ratio, resulting in a more natural product.
F. Cho-Ah-Ying et al., "Adsorptive Removal of Sulfur from Canola Oil,"Fat. Sci. Technol., No. 4, pp. 132-5 (1991), describes an investigation of physical adsoprtion of sulfur using alumina, alumina-silicate, diatomaceous silica and TriSyl.RTM. silica gel (Davison Division, W. R. Grace & Co.-Conn.) in conjunction with bleaching earths. The article reports that compared to the unactivated TriSyl.RTM. silica gel, the activated adsorbent (dried at 240.degree. C. for three hours) had a higher capacity for adsorbing Raney.RTM. nickel sulfur at all concentrations used. For that reason, Cho-Ah-Ying opted to use the activated silica gel adsorbent throughout the experiments. The article reports that the addition of 2 or 4% alumina, alumina-silicate, diatomaceous silica and silica gel (presumably the unactivated form) did not further improve the removal of sulfur.